The detection of very low level voltages, i.e., in the order of 10.sup.-15 volts, has been made possible by the development of supercurrent tunneling across thin insulating barriers as first published by B. D. Josephson in "Possible New Effects In Superconducting Tunneling," Physics Letters 1, Page 251, 1962. The Josephson Effect is a superconducting phenomenon and occurs only at temperatures below a few degrees Kelvin, e.g., 4.5.degree. K., and is used to measure voltages, magnetic fields, magnetic field gradients and magnetic susceptibilities from below audio frequencies to electromagnetic radiation frequencies in the microwave and far infrared. See the article "Josephson Junction Detectors," J. Clarke, Science, 21 June 1974, Volume 184, No. 4143, Pages 1235 through 1242.
More recent studies have been directed toward quantum interference effects in a superconducting ring containing one or two Josephson junctions. These studies have led to the development of the DC and RF SQUID (Superconducting Quantum Interference Device) and the SLUG (Superconducting Low-inductance Undulatory Galvanometer). See the article "Tunnel Junction DC SQUID: Fabrication, Operation and Performance," J. Clarke, et al, Journal of Low Temperature Physics, Volume 25, Nos. 1/2, 1976, Pages 99 through 144. Cryogenic gradiometers, because they use two loop patterns to measure the difference in magnetic field intensities between two points or areas in space, are subject to imbalance primarily due to the differences in loop size and pattern. In the past, balancing has been achieved by adjusting movable superconducting vanes to appropriately distort the magnetic field in the vicinity of the sensor loops. This requires mechanical linkages to adjustment screws external to the dewar. With this procedure, balance is difficult to maintain during thermal cycling and in the presence of mechanical vibrations. The present invention is directed toward an apparatus for and a method of providing balancing without mechanical connection external to the dewar and with permanent mechanical bonding not susceptible to change under mechanical vibration or thermal cycling.